Ferroelectric thin films of Pb(ZrxTi1-x)O3 (lead zirconate titanate or “PZT”) are being developed for next generation electronic devices such as non-volatile and optical memories, micro switches, micro-actuators, micro-positioners, micro sensors, energy generators, etc. The progress made in fabricating high quality PZT films has opened a new area for applications in biomedical devices, especially the possible integration of high quality PZT films with silicon technology. The growth of PZT thin films has been realized by various methods; a method known as the “sol-gel” method is likely to be a dominant growth technique because the sol-gel method is a simple process that meets device fabrication requirements, such as relatively low processing temperature, large area coating ability and ease of composition control (see R. W. Schwartz, J. A. Voigt, B. A. Tuttle, D. A. Payne, T. L. Reichert and R. S. DaSalla, “Comments on the effects of solution precursor characteristics and thermal processing conditions on the crystallization behavior of sol-gel derived PZT thin films,” J. Mater. Res., Vol. 12, no. 2, February 1997, pp. 444).
When promoting the fabrication of a PZT film on a silicon (Si) substrate, which involves coating with a precursor solution and subsequent annealing, it is known to be important to control the formation of a desirable phase in the PZT film, called the “perovskite phase”. It is known that the diffusion of lead (Pb) from the PZT film to the substrate occurs during the annealing of the PZT and can easily induce the formation of an undesirable phase in the PZT film, called the “pyrochlore phase”, at the PZT film/substrate interface (see R. A. Roy, K. F. Etzold, “Substrate and temperature effects in lead zirconate titanate films produced by facing targets sputtering,” J. Mater. Res. Vol. 7, no. 6, June 1992, pp. 1455).
High temperature processing can also cause lead volatilization, which has been shown to result in the formation of undesirable PZT phase and microstructure. For instance, conventional 2-methoxyethanol derived PZT film without any seeding layer has been shown to result in a “rosette” microstructure with micron size non-uniform grains (see J. Lee, C. Kim, D. Yoon, C. Choi, J. Kim, K. No, Jpn. J. Appi. Phys. 33 (1994) 260 and C. Kwok, S. B. Desu, “Low temperature perovskite formation of lead zirconate titanate thin films by a seeding process,” J. Mater. Res., vol. 8, no. 2, February 1993, 339). The undesirable PZT phase and microstructure have been shown to degrade the electrical properties of the PZT film (see K. Sreenivas, I. Reaney, T. Maeder and N. Setter, “Investigation of Pt/Ti bilayer metallization on silicon for ferroelectric thin film integration,” J. Appl. Phys., Vol. 75 (1994), 232-239).
The challenge of integrating PZT films with a silicon substrate is primarily the development of a PZT fabrication process compatible with the silicon without significant formation of the undesirable phase.
More recently, studies have been performed to reduce the processing temperature and improve the ferroelectric properties of the films fabricated using the sol-gel method. Kwok and Desu presented a novel PZT deposition method to include a lead titanate (PbTiO3 or “PT”) seeding layer between the PZT film and a substrate. This seeding layer can offer nucleation sites (i.e., locations where the conditions for the formation of crystals, is favorable) and reduce the activation energy for crystallization of PZT thin films, thereby lowering the perovskite formation temperature (see C. Kwok, S. B. Desu, “Low temperature perovskite formation of lead zirconate titanate thin films by a seeding process,” J. Mater. Res., vol. 8, no. 2, February 1993, 339). The perovskite formation temperature of PZT alone is known to be about 900° C., while the perovskite transformation temperature of a PZT/PT structure is only 750° C. (see U.S. Pat. No. 6,507,060 to Ren et al.). Ren et al. have also demonstrated that PT/PZT/PT structures can be formed at a much lower processing temperature of around 600-700° C. (see T. L. Ren, L. T. Zhang, L. T. Liu and Z. J. Li, “Electrical properties of a silicon-based PT/PZT/PT sandwich structure,” J. Phys. D: Appl. Phys., Volume 33, Number 15, 7 August 2000, L77).
The use of conductive oxide electrodes that can easily adhere to SiO2/Si substrates without the need for additional adhesion layers can also be shown to provide excellent resistance to polarization fatigue (Y. K. Wang, T. Y. Tseng and P. Lin, “Enhanced ferroelectric properties of Pb(Zr0.53Ti0.47)O3 thin films on SrRuO3/Ru/SiO2/Si substrates,” Appl. Phys. Lett., Vol. 80, Issue 20, pp. 3790, May 20, 2002). However, the results reveal that the conductive oxide electrodes lead to a large increase in leakage current and an increased susceptibility to early dielectric breakdown when compared with the use of platinum electrodes. Kim et al. reported the effects of a PT layer on Pt/RuO2 hybrid electrodes and showed control of the second phase (Pb2Ru2O7-x) with improved ferroelectric properties (S. H. Kim, Y. S. Choi, C. E. Kim, D. Y. Yang, “The effects of PbTiO3 thin template layer and Pt/RuO2 hybrid electrode on the ferroelectric properties of sol-gel derived PZT thin film,” Thin Solid Films, vol. 325, issue 1, pp. 72-78, 18 July 1998).
While some gains have been made, further reductions in the processing temperature and further improvements in the ferroelectric properties of PZT film structures may be seen as beneficial.